Cable television systems are well known in the art. Through such systems, a recipient's television receiver may be connected to a cable that connects to the cable company's transmitter. A plurality of television signals are broadcast in the radio frequency range through the cable.
Attempts have been made to send additional messages through such cable systems. For instance, in some prior art systems, a written message appears on the screen of the recipient's receiver. This message may either contain all of the pertinent information that the cable company intends the recipient to receive, or it will instruct the viewer to turn to a specific channel. The intended information will then be viewable on that channel.
Such a system has certain obvious drawbacks. Perhaps foremost, the receiver must actually be on. If not, the intended recipient will not be aware that the cable company has transmitted a message.
Because of such drawbacks, a cable systems have not been particularly exploited in this regard. Nevertheless, there exist many intances when such a message carrying capacity could be well utilized.
For instance, such a system could be used to deliver severe weather warnings to all subscribers in a threatened area. Civil defense messages or environmental health warnings could be similarly delivered.
Other types of messages could be delivered as well. For instance, members of a volunteer fire department could be alerted to the existence of a condition requiring their skills.
The prior art systems are ill-suited for these purposes because they require that the recipient's television receiver be on, and also because they include no means to limit the audience that may receive the message. There exists a need for a communications system suitable for use with a cable television system that will operate even when the recipient's television receiver is deactivated. Such a system should be capable of decoding certain transmission data to thereby allow appropriate messages to be delivered to the intended audience and inappropriate messages to be silenced. Furthermore, such a system should insure termination of the message mode when the message has been completed, a recipient should be able to selectively monitor the message channels if desired and a recipient should be able to terminate a particular message if desired.